The present invention relates to a pneumatic tire and, more particularly, an improved pneumatic tire, the improvement relating to the rolling resistance and the resistance to groove crack generation of the tire.
To minimize the rolling resistance of pneumatic tires, lately there have been propositions made in the tire industry to provide a two-layer structure to the tire tread rubber, comprising a cap rubber layer and a base rubber layer. The propositions are made out of consideration of the fact that the tread rubber tends to exhibit contradictory characteristics with respect to the rolling resistance and the wet skid characteristics of tires, and they are made in an attempt to cancel such tendency of the tread rubber by way of using a rubber capable of exhibiting a remarkable resistance to wet skid for or in the ground-contacting part of the tread rubber, namely the cap rubber layer, while using a rubber capable of providing a low rolling resistance for or in this part of the tread rubber which does not contact the ground or pavement, namely the base rubber layer.
Based on the technical concept of the propositions referred to above, it falls that if, in the thickness of the tread rubber, the proportion of the base rubber layer is increased, by this the rolling resistance can be lowered.
According to the result of experiments conducted by the present inventors, however, it has come to be known that when the proportion occupied by the base rubber layer in the thickness of the tread rubber is increased, the thickness of the cap rubber layer is indispensably reduced at the bottom of grooves forming a tread pattern on the tread surface of the tire, whereby the tire becomes likely to with ease undergo during its running a so-called groove crack trouble, a phenomenon of generation of a crack at the bottom of tire pattern grooves, which is more likely particularly where there is a large difference in the physical properties of rubber, particularly modulus thereof, between the cap rubber layer and the base rubber layer.